In the case of stator windings of dynamoelectric machines such as, for example, turbogenerators, stator winding bars are used that are placed into appropriate grooves in the stator and affixed there. In order to reduce eddy currents, the stator winding bars consist of a plurality of insulated strands that (as shown in FIG. 5) are combined to form stacks 26, 27 arranged in parallel and that are then surrounded by an outer insulation. In order to reduce circulating current losses in the strands, the strands are transposed (Roebel transposition) according to a prescribed pattern (magnitude of the angle) within the active length (in the active part) of the stator winding bar. A stator winding bar formed in this manner is referred to as a Roebel bar.
A standard transposition of the kind known from U.S. Pat. No. 2,821,641 is shown schematically in FIG. 1. The stator winding bar 10 shown in FIG. 1 comprises two stacks, each with ten strands 11. In order to clearly depict the transposition, a strand—designated with the reference numeral 12—is drawn with thicker lines. The stator winding bar 10 is supported with an active part 8 having a length L in a winding groove (not shown here) of the stator. The strands execute a transposition of 540° over the length L of the active part 8, that is to say, each strand has completed a rotation of 540° around the longitudinal axis of the bar in this area. The stack change of the strands 11 needed for the transposition is made possible by appropriately crimping the strands. The transposition per unit of length of the stator winding bar can be larger or smaller. If the transposition per unit of length increases, the crimping distance k decreases (FIG. 1), that is to say, the distance between the crimping sites of strands that are adjacent in the stack. The active part 8 is delineated on both sides by winding heads 9 in which the strands 11 are untwisted.
With the known stator winding bar 10 from FIG. 1, the active part 8 is divided into three areas A, B and C in each of which a transposition by 180° is provided, resulting in a transposition of 3×180°=540° over the entire length L of the active part 8. The two outer areas A and C are the same length and they each extend over one-fourth of the length L of the active part 8. The middle area B extends accordingly over half the length L/2 of the active part.
In the transposition shown in FIG. 1, due to the non-transposed bar parts in the winding head, unequal current distributions still nevertheless occur in the strands, since circular currents (also called circulating currents), which are caused by the magnetic field in the winding head, form in the loops formed by strands and the ears at the ends of the bars. Thus, U.S. Pat. No. 3,614,497 already proposed compensating for the effects of the non-transposed bar parts in the winding head by incorporating so-called voids (that is to say, non-transposed areas). The appertaining transposition diagram is shown in FIG. 2.
The stator winding bar 13 shown there, which again consists of two stacks, each with ten strands 14 and 15, is once again divided over the length L of the active part 8 into the three areas A, B and C in each of which a transposition of 180° is provided. In this case, once again, the lengths of the areas A, B and C amount to L/4, L/2 and L/4. Unlike the solution shown in FIG. 1, however, a segment 16, 17, 18 is inserted in the middle of each area, no transposition occurring in said segments 16, 17, 18. The crimping distance k is correspondingly shortened in the other segments of the areas A, B and C.
The introduction of the voids or non-transposed segments 16, 17 and 18 brings about a reduction in the undesired circulating current effects, but it does not totally eliminate them.